Sturge Weber Syndrome (SWS) is a sporadic, congenital, neuro-cutaneous disorder characterized by a port-wine stain (capillary vascular malformation) affecting the skin and abnormal capillary venous vessels in the leptomeninges of the brain and choroid, leading to glaucoma, seizures, stroke, and intellectual disability. In 1987 Rudolf Happle hypothesized that isolated port-wine stains and SWS are both due to somatic mutation of the same unidentified gene, with the severity and extent of presentation determined by the developmental time point when the somatic mutation occurred. We recently performed genome-wide sequence analysis of affected and unaffected tissue from SWS patients to test Happle's hypothesis. We discovered the identical somatic mutation in GNAQ in nearly all SWS and isolated port-wine stain samples, leading to p.R183Q in the encoded protein, Gaq, a G protein subunit modulating a wide spectrum of downstream signaling pathways. Our preliminary studies suggest this is a gain-of-function mutation, activating one or more downstream pathways. Our data also indicates that this somatic mutation is present in endothelial cells of the SWS affected tissue. In this proposal we will functionally characterize the effects of this somatic mutation. Using an endothelial cell line expressing an inducible form of the mutant Gaq, we will perform an unbiased signaling screen to detect the authentic downstream target(s) of the mutation in the appropriate cellular context. In collaborative with our clinical colleagues, these signaling pathways will be validated by immunostaining affected tissue samples from SWS patients. Using established in vitro assays we will investigate the effect of the mutation on discrete endothelial cell functions related to angiogenesis. Finally, we will determine the phenotypic effects of the GNAQ somatic mutation in vivo by expressing the mutant transcript during zebrafish development. This work represents the first functional validation of the GNAQ somatic mutation in SWS, laying the groundwork for future studies, while holding significant translational potential in the near term. The somatic mutation may activate downstream signaling pathways that have already been targeted with small molecule inhibitors. Thus, this exploratory R21 may lead to new and testable therapy for SWS.